(1) Field of the Invention
This invention concerns a method for the chemical reduction and stabilization of reducible metals that comprises mixing a reducible metal with one or more elemental metals and a chemical reducing agent to effect the reduction of the reducible metals. A stabilizing agent is then added to the solution to stabilize and in some cases precipitate the reduced metals. The resulting stabilized reduced metals exhibit a very low leaching rates.
(2) Description of the Art
Many methods are known in the prior art for reducing reducible metals. One well known method involves the reduction of reducible metals with a solid metal. This method is inefficient however, requiring the use of metal fines and a well mixed solution. Another metal reducing method involves the combination of a reducible metal with a reducing agent such as iron sulfate powder. The reduction can be accomplished in the solid or liquid phas This method, however, requires a large excess of reducing agent in order to sustain the reducing reaction.
U.S. Pat. No. 4,194,973 discloses a method for decolorizing a solution containing dyes, and specifically chromophoric compounds such as aryl azo- or aryl nito- containing dyes. The method includes combining the dye solution with a source of Fe(II) and iron at metal reduction conditions. The chromophoric compounds disclosed contain Cr(III or VI). Furthermore, the reduced metals are precipitated from the solution and recovered and the resulting decolorized solution is disposed of. The patent discloses that the reducing agent attacks the dye portion of the compound.
A paper, titled "Iron Conditioning of Sludge from Foundary Furnace Emission Control Systems for Elimination of EP Toxicity", W. A. Stephens, et al., Proceedings of the 39th Industrial Waste Conference; Madison, Wis. (1985) pp. 511-14 discloses a study performed by the EPA in which ferric sulfate or iron filings were separately combined with leachable lead and cadmium containing wastes to reduce lead and cadmium leaching rates.
An article by P. D. Turpin, et al.; Trans. Amer. Foundrymen's Soc.; 93: 737-40 (1983) describes methods for decreasing lead and cadmium leaching rates of foundry solid waste. The methods include, in the alternative, treating the leachable solid wastes with ferric hydroxide, metallic iron, phosphate, magnesium hydroxide, or lime. The article discusses various methods used to reduce waste leaching rates including absorption, precipitation, chemical reduction, and pH solubility control. The chemical reduction method disclosed is performed using iron filings.
An article by D. C. Nagle, et al., Trans. Amer. Foundrymen's Soc.; 91: 715-20 (1983) discloses a number of methods for treating hazardous wastes generated by foundries. The methods include chemical treatment using sodium phosphate and controlling waste pH to keep the waste metals from solubilizing. The article generally discusses solidification and encapsulation of hazardous wastes in cement.
An article by L. E. Eary, et al.; Environ. Sci. Technol; 22:972-77 (1988) discloses the optimum conditions for chromate removal from oxygenated wastes. The method describes reducing Cr(VI) to Cr(III) using ferrous sulfate. Another article by Eary, et al.; Am. J. Sci.; 289: 180-213 (1989) discloses that Cr(VI) can be reduced to Cr(III) by combining a chromate solution with ferrous iron.